1. Field of the Invention
The present invention relates generally to a power converter protection device and more particularly to a protection device for a power converter which employs gate turn-off thyristors which reduces overvoltages from external surges.
2. Discussion of the Background
In order to avoid damage to the switching elements in a power converter, it has been common to provide a protection device. When the power converter has employed gate turn-off thyristors (GTOs), a diode clipper may be utilized as a protection device, as shown in FIG. 9.
The power converter 7 is made up of a plurality of arm modules 1 each of which contains GTOs or similar switching elements. Terminal 17, connected to the current side of the converter, serves as connection to a direct current power supply 8. The power converter is connected on the other side through alternating current terminals 18. The diode clipper 6 is also connected to each of the AC terminals. Diodes 2 are provided on either side of the connection to the AC terminals for rectification. Capacitor 3 is provided to absorb external surges. Resistor 4 limits the current applied thereto and resistor 5 acts as a discharge path.
FIG. 10a shows an example of the arm modules 1 used in FIG. 9. GTOs 11a and 11b act as switching elements, diodes 12a and 12b act as flywheel diodes and elements 13 are snubber circuits. Reactor 14 is provided to keep the rate of change of the current low. Resistor 15 consumes the energy of the reactor while diode 16 restricts the direction of the current flow through the resistor. The configuration of the snubber circuit 13 is shown in FIG. 10b. The circuit includes capacitor 21, resistor 22 and diode 23.
Normally, in a GTO power converter it has been conventional practice to prevent overvoltages from external surges entering from the AC side by using a circuit such as diode clipper 6 to protect the switching elements. The operation of this diode clipper is described below.
The capacitor 3 is charged up to the peak voltage of the alternating current circuit by way of the rectification diode 2 and normally no current flows into the capacitor 3. However, when an overvoltage occurs from an external surge on the AC side, the rectification diode 2 becomes conductive and the low impedance of capacitor 3 is connected to the AC side to suppress the overvoltage. Accordingly, these switching elements are protected from the overvoltage.
When a multiplex power converter is utilized, the diode clipper must assume the configuration shown in FIG. 11. A diode clipper 6 must be provided for each converter 7 in order to provide overvoltage protection. Transformer 31 acts as a multiplexer for the power converters.
Thus, in this type of arrangement it is necessary to provide one overvoltage device for each converter. If the power converter capacity is increased utilizing power converter multiplexing, it is necessary to provide an overvoltage device for each multiplex channel. Thus, the size of the equipment becomes large and the cost of the device becomes high.